In magnetic media, information is stored in binary form—one or zero, depending on which way the electronic spins are aligned in a given section of the medium. Recently, however, magnetic vortices have drawn scientists toward a new possibility: multibit storage in which each logic unit has four states instead of two and can store twice the information. Each tiny magnetic whirl has a polarity that can point up or down and a circulation that can be oriented clockwise or counterclockwise. Previous studies have shown that the polarity can be flipped on command. Now, using time-resolved magnetic soft x-ray microscopy at the ALS, researchers have shown for the first time how to use pulsed magnetic fields to reverse the circulation.

Taming the Whirlwind

Although magnetic vortices may seem similar to tornadoes and whirlpools, they are dynamically very different. The electronic spins that constitute a magnetic vortex are not actually swirling around the core. What they can do is flip in place, and the core—defined as the area where spins point out of the plane—can be moved around on the disk or pushed off the edge by external forces, in which case a new core will form (see the accompanying video). Left alone, however, magnetic vortices are structurally and thermally very stable and form readily in soft magnetic materials of the sort useful for magnetic random-access memory (MRAM) devices.

Interestingly, the formation of such vortices was at first considered a problem for these devices because they led to signal deterioration and information loss. But our ability to fabricate and study materials at the nanoscale has led to the idea that these vortices can be harnessed to encode bits of information. Consumers spent 15 percent of home energy on gadgets in 2009, adding more gadgets all the time. The research described here by Uhlíř et al. is an important step toward having the potential to significantly improve the performance of these gadgets while at the same time being much more energy efficient.

Magnetic vortex structures establish a four-bit magnetic storage/logic unit, represented by two levels of polarity (up/down) and two levels of circulation (clockwise/counterclockwise).

Simulation of controlled circulation switching in a 100-nm-wide, 20-nm-thick disk with a wedge-like asymmetry. Reversing the circulation requires driving the vortex core completely off the edge of the disk, causing the vortex to collapse and reform.

In magnetic vortices confined to tiny metal disks a few nanometers in diameter, the electron spins seek the lowest possible energy. Adjacent spins pointing in opposing directions cost energy. To avoid this, the electrons tend to arrange themselves into a circle, pointing either clockwise or counterclockwise around the disk. In the core of the vortex, however, where the circles get smaller and smaller and neighboring spins would inevitably align antiparallel, they tend to tilt out of the plane, pointing either up or down. So each disk has four bits instead of two—left or right circulation and up or down polarity of the core—but each orientation must be independently controllable.

Previously, researchers working at the ALS had found that a weak oscillating magnetic field in the plane of the nanodisk could reverse the polarity of the vortex core. As the gyrating core is nudged away from the center of the disk, successive magnetic waves move the core faster and faster until its polarity flips to the opposite orientation, all in less than a nanosecond. Now, a team of researchers from the Czech Republic, San Diego, and Berkeley, working at ALS Beamline 6.1.2, have demonstrated that similar methods can be used to reverse the direction of circulation.

Time-resolved soft x-ray transmission microscopy at ALS Beamline 6.1.2 allows researchers to make direct images of how the strength and duration of trains of electric and magnetic pulses affect the circulation of a magnetic vortex. No other method can provide similarly comprehensive information about the fastest dynamics of magnetic states on the nanoscale. These instruments are unique and serve the whole vortex community, worldwide.

In this work, the researchers demonstrated, for the first time, that nanosecond pulses of an external magnetic field can reverse the direction of circulation, and that the threshold field strength of the pulses is about half that required for a static field. The results are supported by both analytical models and micromagnetic simulations, showing that both the time and field-switching scales strongly depend on the disk geometry.

Magnetic transmission soft x-ray microscopy images showing the vortex circulation in disks of different widths (510 and 250 nm) and by static vs pulsed magnetic fields. The arrows on the right of each image indicate spin circulation. Arrows and labels below the images show the polarity and magnitude of the applied magnetic field, as well as pulse duration where applicable.

The disks were all tapered, with diagonal slices off their top surfaces that served to control the final orientation of circulation. But thickness and diameter were also important factors: the smaller the disk, the better. Thick disks (30 nanometers) over a thousand nanometers in diameter were slow, taking more than three nanoseconds to switch circulation. But disks only 20 nanometers thick and 100 nanometers across could switch orientation in less than 0.5 nanoseconds.

Much remains to be done before the four-value multibit becomes practical. Polarity can be controlled, and circulation can be controlled, but so far they can't be controlled at the same time. Plans for doing this are in the works. The researchers are also looking at ways to control spin with temperature and voltage, at how to completely decouple spin from charge currents, and even at ways to couple chains of nanodisks together to build logic devices—not just for memory, but for computation.

Research funding: European Regional Development Fund, Grant Agency of the Czech Republic, and the U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES). Operation of the ALS is supported by DOE BES.